Supercritical extraction of tar sands as well as oil shale are known approaches to recovery of organic materials or hydrocarbons from solids containing such material. In the case of tar sands the bitumenous organic material is accessible to solvents whereas in the case of oil shale the finely divided oil shale generally is subjected to supercritical conditions of temperature and pressure while in contact with a solvent in the order to produce extractable organic material from the kerogen in the oil shale. Another example where solids under high pressure occur is a coal conversion process, such as supercritical coal extraction.
In each instance of extraction of organic material present in solids under extremely high pressure one of the key problems that arises consists in the difficulty of discharging the high pressure solid material.